10 Bizarre Things Your Body Does While You Sleep

They're kind of amazing, actually.

Sleep — such a peaceful word, isn't it? But while you're dozing, your body's systems are doing lots of crazy things that will blow your mind. Don't worry: They're all normal. Still, you may never think of hitting the hay the same way after reading this.

1. Your body temperature drops.
Just before you fall asleep, your core body temperature begins to decrease, says Michael Breus, PhD, a clinical psychologist and sleep specialist, and author of The Sleep Doctor's Diet Plan: Lose Weight Through Better Sleep. This drop signals to your brain to release melatonin, which affects your circadian rhythm (or sleep/wake cycle) and tells your body it's time for bed. Your temperature is lowest around 2:30 A.M., so if you're able to, program your thermostat to rise one degree at that time for an hour or two. Otherwise, you may find yourself stealing your spouse's covers for extra warmth.

2. You lose weight.
One reason you should always step on a scale in the morning, not in the evening: You lose water through perspiring and breathing out humid air during the night, according to Dr. Breus. This happens during the day too, but eating and drinking while you're awake negates any weight loss. If you're sleeping just four or five hours per night, you could be canceling out whatever smart diet and exercise choices you're making during the day. To whittle your waistline, get at least seven hours of sleep per night.

3. You get taller.
You won't exactly wake up feeling like the Jolly Green Giant, but you do gain height while you sleep. "The discs in your spine that act as cushions between the bones rehydrate and get bigger because the weight of your body isn't pressing down on them, like it is when you're standing," says Dr. Breus. "If you have a firm mattress, sleeping on your side in the fetal position may be best for getting taller because it decreases the load on your back."

4. Your blood pressure and heart decrease.
When you're resting, your body doesn't need to work as hard or pump as much blood, so these systems slow down. Blood pressure needs to dip at night so your cardiac muscle and circulatory system have time to relax and repair, says Dr. Breus. It's especially important for people with high blood pressure to get at least seven hours of sleep to experience that temporary drop — it reduces the risk for heart disease. And if you have sleep apnea, get treatment right away because that condition can increase nighttime blood pressure, says Dr. Breus.

5. Your muscles are temporarily paralyzed.
Sounds scary, but it's actually what keeps you from acting out your dreams, says Lisa Shives, MD, an internist and sleep medicine specialist and the medical expert for SleepBetter.org.

6. Your eyes twitch.
During REM (rapid eye movement) sleep, your eyes dart from side to side, not that scientists know why exactly. Dreams occur during REM sleep, so it can be disconcerting to wake up during this deep — not light — sleep stage. You might feel most refreshed if you wake up right after you cycle through all the sleep stages, with REM occurring toward the end. Though it varies from person to person, one sleep cycle usually lasts 90 minutes, so try sleeping in intervals of 90 minutes. For example, you may find it easier to awaken after sleeping for 7.5 hours (five cycles) than after 8 hours (5 1/3 cycles).

7. You get sexually aroused.
Just as men get erections during REM sleep, women become sexually stimulated then, too. And no, it's not tied to whether you're having that Brad Pitt dream again. Your brain is more active during REM sleep (since you're dreaming), so it requires more oxygen — as a result, blood flow all over the body increases. "There is natural clitoral engorgement because blood rushes to that area and causes swelling," says Dr. Shives. Does that make you more likely to orgasm if you engage in middle-of-the-night nooky? Scientists aren't sure, but go ahead and experiment!

8. You're more likely to have gas.
You won't be happy to hear this, but during the night, your anal sphincter muscles loosen slightly, making it easier to let out a toot or two. Luckily, your sense of smell (and your spouse's) are reduced while you sleep — that's why fire alarms were invented, because it's hard to smell smoke while you're snoozing. So even if you experience flatulence, rest assured: Nobody is likely to notice.

9. You may have a full-body spasm.
"As people fall asleep, many of them experience a full-body jerk, and it's totally normal," says Dr. Shives. As many as 70% of people experience this phenomenon in which muscles suddenly contract (the technical name for it is "hypnic jerk"). Some experts think that these spasms may have to do with anxiety and/or an irregular sleep schedule, while others think they're unavoidable. So if you like to snuggle with your spouse as you wind down in bed, be sure to pry yourselves apart before you're both out cold, or else you may accidentally shake each other awake.

10. Collagen production in your skin increases.
Collagen is a protein that strengthens blood vessels and gives skin its elasticity. When you're asleep, you're in a fasting state, so growth hormone is released to tell your fat cells to release energy stores — as it turns out, growth hormone also stimulates collagen growth. "Since collagen production spikes while you sleep, moisturizing facial creams that contain retinols and retinoids are best to use before bed because these products boost collagen turnover, combat pigment problems, and fight fine lines and wrinkles," says Melanie Palm, MD, a dermatologist, an assistant clinical professor at the University of California, San Diego, and a staff physician at Scripps Encinitas Memorial Hospital.

Jane BianchiJane Bianchi is a writer and editor with more 13 years of experience specializing in health; she formerly worked as a health editor at Family Circle, and her work has appeared in Men’s Health, Women’s Health, Esquire, and more.

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